For controlling the flow of a fluid to be sprayed, it is common for spray guns to have a valve assembly that includes a valve plug and an attached valve stem. The valve plug is operable for (a) seating with the interior of a fluid spray nozzle or valve seat adjacent the nozzle""s fluid exit for thereby prohibiting fluid exit, and (b) retracting from this nozzle or valve seat when it is desirable to spray the fluid. Typically, the valve plug has a generally spherical configuration and is attached to an end of the valve stem via an intermediate component denoted as a xe2x80x9cholderxe2x80x9d herein. Accordingly, when the valve assembly is operably incorporated into a spray gun, a trigger for the spray gun is used to pull the valve plug from its seat or seal with the interior of the fluid nozzle for thereby allowing the fluid to exit the spray nozzle. Since many fluids to be sprayed are abrasive, the valve plugs are manufactured from abrasion resistant materials such as alloys of tungsten carbide. Moreover, since tungsten carbide is relatively expensive (and difficult to machine) and other components of such valve assemblies are not subject to the extreme abrasive wear of the valve plug, the valve shaft and any other components of such valve assemblies are typically manufactured from less expensive materials such as stainless steel.
Heretofore, it has been difficult to reliably secure such tungsten carbide alloy valve plugs to the other components of the valve assemblies. In particular, a high proportion of such valve assemblies break long before there is significant abrasive wear. That is, the prior art techniques for securing such valve plugs to the other components of their corresponding valve assemblies has been substantially inadequate for inexpensively producing highly reliable valve assemblies. For example, one common technique for securing valve plugs within their corresponding valve assemblies is to braze the valve plug onto an end of the valve stem or onto an intermediate valve plug holder. However, a brazing process results in unsatisfactory valve assemblies for the following reasons (1.1) through (1.3) below:
(1.1) Brazing results in relatively unreliable joints between the valve plug and the other components of the corresponding valve assembly. For instance, during manufacturing of such brazed valve assemblies, stress testing of the brazed joints can have a failure rate as high as 50% of the valve assemblies produced.
(1.2) The solder used as brazing material is relatively soft and the resulting soldered joint for securing the valve plug is prone to failure during use. A typical (spherical) valve plug may have a diameter of approximately 0.125xe2x80x3 and the brazed, soldered joint contacting the valve seat is likely to be much less than a single hemisphere of the valve seat. Additionally, it is not uncommon for spray guns to have fluid spray pressures of 3,000 psi and accordingly, a force of approximately 100 lbs. may be required to disengage the valve plug from its seal with the spray fluid nozzle. Moreover, a corresponding opposite force may be applied to the valve plug as it reseals with the fluid nozzle. Thus, the soldered joint may fail before the valve plug is abraded to the extent that it fails to fully seat with the fluid nozzle.
(1.3) Brazing is prone to spatter solder on the valve plug. This spatter may result in the valve plug not fully sealing with the spray fluid nozzle if not removed. Accordingly, it is not uncommon in the manufacture of such prior art brazed valve assemblies to manually examine and remove such spattering.
Thus, it would be desirable to have a process for more reliably attaching valve plugs to their corresponding valve assemblies. In particular, it would be desirable to attach such valve plugs to their valve shafts by welding rather than brazing.
Additionally, the securing of the valve stem within a resulting valve assembly has typically required that an end of the valve stem be secured within a bore of an intermediate component (i.e., the xe2x80x9cholderxe2x80x9d) between the valve plug and the valve stem. Further, in order to withstand the forces as described in (1.2) above, the valve stem end may be mashed, cold headed or otherwise flared so that this end cannot easily exit the bore. Subsequently, this stem end may be further affixed within the holder by another brazing process.
Thus, it would be additionally desirable to have a process for more easily securing the valve stem to the holder. In particular, it would be desirable to secure the valve stem to the holder by a simple welding process, wherein there is no need to flare the end of the valve stem prior to welding.
The present invention is a novel spray gun valve assembly and process for manufacturing the same. In particular, the present invention uses resistance welding techniques for welding a valve plug of such an assembly to one of: a valve stem, and a holder that functions as a intermediate component to which both the valve plug and the valve stem of the assembly are secured. Since the valve plug and valve assembly may be composed of such dissimilar materials as tungsten carbide alloys (for the plug), and stainless steal (for the valve stem), resistance welding, according to the present invention, provides an effective and reliable bond to withstand the seating and unseating forces to which such valve assemblies are subjected within spray guns.
Moreover, the present invention also includes a novel method for securing the above identified holder to the valve stem, wherein the two are secured together both by a resistance weld and a lock generated by deforming a portion of the valve stem. Furthermore, this novel securing method is capable of being used to secure together a wide variety of male and female components.
Other features and benefits of the present invention will become evident from the accompanying drawings and the detailed description herein below.